The present invention relates to a timing recovery circuit and, more particularly, to a circuit for recovering timing signals out of split-phase or Manchester coded data.
There are both analog and digital versions of such a timing recovery circuit. An example of analog type timing recovery circuit is described in THE BELL SYSTEM TECHNICAL JOURNAL, Vol. 58, No. 1, January 1979, pp. 139-143. An analog type timing recovery circuit extracts the timing component of a frequency equal to half the bit rate of a preamble or dotting section of "1" and "0" added before the data, and achieves synchronization by using the extracted timing component. This kind of analog type recovery circuit has the disadvantage of complex circuitry, which prevents ready large-scale integration.
A digital type timing recovery circuit, as will be described afterwards with reference to some of the accompanying drawings, readily permits large-scale integration. However, it has its own problem that, if either "1" or "0" emerges consecutively in the data, the varying points of the data, which should be at the trailing edge of the clock, will be displaced by 180 degrees, making synchronization impossible. To avoid the phase displacement of the regenerated clock, the conventional digital timing recovery circuit controls an oscillator for timing synchronization only during the dotting section and the synchronized oscillator is maintained during the data section. In this case, however, the stability of the oscillator may pose a problem. With an insufficiently stable oscillator, the frame length of the data cannot be made great enough, resulting in a poor efficiency of channel utilization. On the other hand, a sufficiently stable oscillator would be too expensive and constitute a disadvantage.